Magnesium oxide coating composition and process

ABSTRACT

MAGNESIUM OXIDE COATING COMPOSITIONS AND PROCESS FOR APPLYING TO SILICON-CONTAINING MAGNETIC STEEL AN ADHERENT COATING WHICH ACTS AS A SEPARATING MEDIUM TO PREVENT FUSING OR WELDING OF COILED SHEET DURING AN ANNEALING PROCESS AND WHICH COATING PROVIDES ELECTRICAL INSULATIING AND CORROSION RESISTANT PROPERTIES. THE COATING COMPOSITIONS COMPRISE A MAJOR PROPORTION OF MAGNESIUM OXIDE AND A MINOR PROPORTION OF ADDITIVES CONTAINING A BORON COMPOUND, A SODIUM COMPOUND, AND SILICA OR A SILICON-CONTAINING COMPOUND.

June 8, 1971 J. F. STEGER ETAL 3,583,887

MAGNESIUM OXIDE COATING COMPOSITION AND PROCESS Filed Aug. 18, 1969 q NQ as B Q f 3 if!) U) Q N w 5 LU 2U \lq (\I :5; w 5 V") q.

I 21 ru 2 0 q 5 m 6 m 5% S 5 6 255 Q 5 5% Q q$ u Q1 O N INVENTORS JOHNF. STEGEFB JAY W PALMER Q M/)/AZW ATYYS United States Patent Office3,583,887 MAGNESIUM OXIDE COATING COMPOSITION AND PROCESS John F. Stegerand Jay W. Palmer, Crystal Lake, 111., assignors to MortonInternational, Inc., Chicago, Ill. Filed Aug. 18, 1969, Ser. No. 850,910Int. Cl. B23b 15/04; H01f N04 US. Cl. 14827 15 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND OF THE INVENTION Field of the invention Electricaltransformers and other types of electrical induction apparatus are madeusing core materials made from steel sheet having soft magneticproperties. Such sheet is prepared by cold-rolling silicon-containingsteel into sheet form, coiling the steel sheet into rolls and thereafterannealing the coiled steel by a controlled heating process to produce agrain oriented structure having desirable magnetic properties.

Magnesium oxide is used extensively as a highly heat resistant separatormedium and protective coating for metal surfaces. It is also used as anelectrical insulator coating for metals, as a gatherer for removingimpurities, such as sulfur and carbon, from thin metal sheets andparticularly as a protective coating for silicon steel. The electricalinsulating coating is understood to be derived from coating magnesiumoxide on steel, which then forms a film or coating containing Mg SiOwhich coating is an effective electrical insulator when annealed.

Description of the prior art According to the present industrialpractice, siliconcontaining steel is cold rolled into sheets,decarburized and thereafter coiled into convenient rolls. Cold rollingdevelops in the steel the potential to form a grain oriented structurewhen the steel is later annealed. The term annealed refers to a processwhereby the steel is heated to about 1200 C. in an essentially purehydrogen atmosphere (or vacuum) under programmed conditions with respectto time and temperature. This results in a growth in size of the steelgrains and also in a specific grain orientation which provides thedesired soft magnetic properties sought. During the annealing process,virtually all of the remaining excess carbon and sulfur content of thesteel is lost. This type of annealing is known in the art as boxannealing.

The steel so produced is in the form of a long, thin, coiled sheet whichmust be uncoiled and thereafter fabricated into the desired shape andsize for its intended use. In the process of fabrication which caninclude, for example, punching, stamping and bending, strains andstresses are introduced in the steel which impair its magneticproperties. Consequently, the strains and stress must be relieved. Thisis accomplished by a process termed stress relief annealing whichcomprises reheating the fabricated steel to a temperature of about 800C. in a ,1

reducing atmosphere for a specified period of time.

3,583,887 Patented June 8, 1971 During the box annealing process wherethe steel is in large coils, in the absence of a suitable separatingmedium, the coiled roll would fuse to itself and could not be unrolled.Conventionally, this is avoided by placing a thin coating of magnesiumoxide on the steel prior to coiling. Further, the magnesium oxidecoating serves to reduce impurities such as carbon and sulfur in steelby chemical reaction. In addition, magnesium oxide provides aninsulating silicate layer by reaction with the silicon in the steel.There is also evidence that some iron silicate is formed. For certainapplications, it is also important to form an efiicient electricalinsulating coating. Thus, for example, transformer cores are constructedfrom thin sheets of soft magnetic steel stacked together to form alaminated body in which each sheet is electrically insulated from itsneighbor. This construction vastly reduces the generation of eddycurrents in the core imposed by an alternating electrical field. Theaverage density of soft iron in the core should be as large as possibleand consequently the insulation on the plates should be as thin aspossible to provide closer stacking of steel plates.

Obviously, short circuits between plates reduces transformer efficiencyand often causes the development of damaging hot spots in thetransformer core. Consequently soft magnetic steel is rated by thenumber of short circuits per unit area, usually expressed in terms ofthe electrical resistance of the insulating layer. This is a standardASTM measurement known as the Franklin Test.

Since stress relief annealing is sometimes conducted in an atmosphere ofimpure hydrogen containing hydrocarbons, moisture and oxygen, thesurface of the steel beneath the insulating magnesium oxide layer isattacked forming inclusions of interstitial carbides and oxides therebyimpairing the magnetic properties of the steel. This quite obviously isa serious problem which demands a particular type of insulation topreserve the necessary magnetic properties.

A further problem arises because magnesium oxide is applied to steelfrom an aqueous suspension or slurry. During the initial stages of thebox annealing process, water volatilizes from an applied coating and canunder ordinary conditions react with the iron on the surface of thesteel forming current conducting iron oxides which impair the electricalinsulating properties sought. Areas on the surface of the steeldisplaying such deleterious iron oxide formations are referred to in theart as anneal patterns. Furthermore, it has been found that magnesiumoxide does not form an intimately bonded tightly adherent coating whichis not easily removed upon contact.

It has been found that magnesium oxide per se fails to provide thedesired high degree of required insulation and magnetic protectionproperties to overcome the noted defects. Accordingly, it would bedesirable to provide magnesium oxide compositions for coating steelwhich compositions are tightly adherent and maintain suitable electricalinsulating properties after box annealing and stress relief annealing,and which also preserve the desired magnetic properties of the steelafter stress relief annealing.

It is therefore an object of the present invention to provide improvedmagnesium oxide compositions for coating silicon steel whichcompositions form electrical insulating barriers that prevent theformation of undesirable anneal patterns believed to be due to ironoxide formations during box annealing.

It is another object of the present invention to provide magnesium oxidecompositions for coating silicon steel which compositions form adherentelectrical insulating coatings which are also capable of preventingattack of the steel surface by impure hydrogen during stress reliefannealing.

It is still another object of the present invention to provide magnesiumoxide coating compositions for coating silicon steel which compositionshave the above attributes and also function as eflicient separator mediafor coiled, rolled steel sheet.

It is a further object to provide a simplified process for coating steelsheet with said magnesium oxide compositions to achieve theaforementioned benefits.

The fulfillment of these and other related objects of this invention maybe more readily appreciated by reference to the following specification,examples, and appended claims.

SUMMARY OF THE INVENTION Broadly, the above objects are accomplished bythe provision of magnesium oxide coating compositions containing a majorproportion of magnesium oxide, and a minor proportion of the additives:

The invention also provides a process for coating steel sheet with anadherent, electrical insulating, corrosionresistant coating comprisingin sequence applying to the surface of said steel an aqueous slurrycontaining the said magnesium oxide coating composition, heating toremove water therefrom, and thereafter annealing the coated steel at atemperature in excess of about 1000 C.

The magnesium oxide employed has a surface area in excess of about 20square meters per gram or an iodine number (iodine adsorption valve) inexcess of about 24 milligrams of iodine per gram of dry magnesium oxide.The surface area of magnesium oxide as referred to herein is determinedby the standard Brunaeur-Emmet-Teller method. The iodine number isapproximately 1 to 1.25 times the surface area in square meters pergram.

The iodine number as referred to herein is determined by the followingprocedure:

METHOD OF IODINE NUMBER DETERMINATION (1) Weight a 2 gram sample ofmagnesium oxide to the nearest milligram.

(2) Transfer to a clear, dry, 200-ml. glass-stoppered bottle.

(3) Add 100:0.2 ml. of 0.100 N iodine in carbon tetrachloride, free fromtraces of sulfur or carbon disulfide.

(4) Stopper the bottle and shake vigorously at ambient temperature in asuitable shaking device for 30 minutes (the test is relativelyinsensitive to temperature so that no temperature controls areemployed).

(5) Allow to settle for 5 minutes or longer and then pipette a 20 ml.aliquot of the clear solution into a 250 ml. Erlenmeyer flask containing50 ml. of 0.03 N potassium iodide in 75 percent ethanol,

(6) Titrate the 20 ml. aliquot with standard 0.05 N sodium thiosulfate.The sodium thiosulfate should be standardized at least once every twoweeks against a standard potassium iodate solution. A sharp end point isobtained without the use of starch indicator.

(7) Calculate iodine number in terms of milligrams of iodine per gram ofsample according to the following equation:

where V is the volume of thiosulfate equivalent to 20 ml. of theoriginal iodine solutionbefore adsorption of iodine by the oxide; whereV is the volume of thiosulfate required by the 20 ml. aliquot afteradsorption; and N is the normality of the thiosulfate solution.

By the term boric acids it is meant to include all of the various knownboric acids, such as for example, but not limited to, metaboric acid,orthoboric acid and pyroboric acids.

By the term magnesium borates it is meant to include the various knownmagnesium borates, such as for example, but not limited to the magnesiumorthoborates, the magnesium metaborates and the magnesium pyroborates.

By the term sodium borate it is meant to include all of the variousknown sodium borates, such as for example, but not limited to sodiumtetraborate, sodium metaborate, and sodium orthoborate. Silica invirtually any form and having a particle less than about 200 mesh may beemployed, such as the product available under the trade name Cabosil.Equally operable are the alkaline earth metal silicates, such as forexample, calcium silicate and magnesium silicate.

Generally, the concentrations of additives, on a molar ratio basis,range as follows:

The molar ratio of magnesium oxide to silica is from about 280:1 toabout 2900:1, preferably from about 1000:l to 2000:1;

The molar ratio of boron (expressed as B 0 to silica is from about 3:1to about 5:1, preferably from about 3 :1 to 4:1;

The molar ratio of magnesium oxide to sodium is from about 2000:l toabout 400011 with about 2800:l being preferred.

In the aggregate the additives (silica, boron and sodium, calculated asexpressed above) in the coating composition may range up to about 2 /2mole percent and are preferably present in amounts up to about 2 molepercent, based on the total moles of MgO and additives.

For coating steel, an aqueous suspension of the above describedmagnesium oxide composition is prepared by mixing with Water to thedesired viscosity and leveling and flow-out characteristics. Generallyfrom about 5 to about 20 Weight percent of the magnesium oxidecomposition, based on water, is satisfactory to provide an aqueousslurry having the requisite viscosity and flow properties suitable forcoating onto steel sheet.

A frit may also be prepared by fusing magnesium oxide with aboron-containing compound, a sodium compound and a silicon-containingcompound as hereinbefore described. The frit may then be comminuted to aparticle size less than 200 mesh and added to Water to form the coatingslurry. In preparing aqueous slurries of the coating compositions, theorder of addition or admixture of the various components of thecomposition is immaterial.

The coating slurry may be applied to the magnetic sheet material by anysuitable means such as by immersion, brushing, or spraying. It has beenfound convenient to use an immersion technique whereby the steel sheetis Passed through a tank containing the coating slurry, The coated sheetthereafter is heated to drive off water and provide a dried layer of thepresent composition. The coated metal sheet is then coiled into a rolland placed in a furnace for box annealing as previously described.During the annealing process, the coating of this invention forms anadherent, electrically insulating, corrosion-resistant layer which alsofunctions as a separator medium to prevent the coiled metal sheet fromsticking to itself.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic diagram of aconventional process for coating the compositions of the presentinvention on steel strip. As therein illustrated, a magnesium oxidecomposition contained in hopper 10 and water in tank 12 are fed via 13and 13a to mixing tank 14 to form an aqueous coating suspension orslurry. The aqueous coating slurry is conducted through line 18 equippedwith valve 20 to coating trough 22. Newly formed, hot steel sheet 23 ispassed in the direction indicated by arrows over roller 24 and underroller 26 through quench tank 28 and thence over roller 25 and throughcoating trough 22 where it acquires a layer of the coating slurry. Thesteel sheet coated with an aqueous slurry of the coating composition isthen passed between adjustable pinch rollers 30 which regulate thecoating thickness and then to heater 32. In the heater, water isvolatilized from the slurry layer on the steel strip to provide anadherent coating. The coated steel sheet is passed over roller 33 andcoiled into suitable lengths at take-up roll 34. The coated coiled steelroll 23A is thereafter passed to annealing furnace 36, and thence tostorage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one preferred form, a steelcoating composition comprising magnesium oxide, boric acid, sodiumhydroxide, and silica is provided in which the molar ratio of magnesiumoxide to silica is about 1500:l, the molar ratio of boric oxide tosilica is about 3.521 and the molar ratio of magnesium oxide to sodiumis about 2800:1. Approximately 10 to 12 Weight percent of thiscomposition is admixed with water to form a slurry suitable for coatingonto steel sheet. Coiled steel sheet which was coated with this slurryand subsequently dried and thereafter box annealed according to theprocedure hereinbefore described was easily unrolled. Further, thesurface of the steel was relatively electrically non-conducting andresistant to attack by corrosive gases or fumes.

In another preferred form a magnesium oxide composition having theaforenoted desirable steel coating properties is provided whichcomprises magnesium oxide, boric oxide, sodium tetraborate, and silicain which the molar ratio of MgO to silica is about 1500:1, the molarratio of boric oxide to silica is about 3.5 :1, and the molar ratio ofmagnesium oxide to sodium is about 2800: 1.

For a more complete understanding of the present invention, reference isnow made to the following specific examples illustrating the improvedproperties of the coating compositions of the present invention.

EXAMPLE 1 A steel coating suspension was prepared by adding 18.0 gramsof MgO (0.45 mole) to 150 ml. of an aqueous boric acid solutioncontaining 0.00105 mole of boron expressed as B To this suspension wasadded 0.00016 mole of sodium hydroxide and 0.0003 mole of silica havinga crystallite size of about 70 A., to yield an aqueous coatingcomposition of about solids containing the following components in theindicated ratios:

Coating Composition A: Molar ratio MgOzSiO 1500:1 B203 :SiO 1 MgOzNa281221 Coating procedure The resultant suspension was subjected toconstant stirring and was coated onto 26 separate silicon steel sheetsor plates, each sheet measuring 3.0 x 15.3 cm. by 12-14 mils inthickness. The coatings were leveled and then dried in air at atemperature of about 500 C. The coated, dried sheets were then stackedone upon another and annealed by heating in a hydrogen atmosphere at atemperature of 1177 C. for a period of 4.3 hours. The annealed sheetswere then cooled, and excess or loose magnesium oxide was scrubbed fromthe surface of each coated sheet by brushing in a stream of flowingwater.

The procedure was repeated except that the ratio of MgOzSiO in thecoating composition was increased to- 296021 as indicated:

Coating Composition B: Molar ratio The conductance of the coatedsurfaces of the sheets coated with Coating Composition A and CoatingComposition B respectively was measured by means of the Franklin test.

Franklin test This test is widely accepted and utilized for evaluatingthe conductance of coated steel sheets. A detailed description is foundin ASTM method A-334-52, Standard Methods of Test for Electrical andMechanical Properties of Magnetic Materials. Briefly, the test iscarried out by passing an electric current through brass contacts whichcover coated areas 0.1 square inch in area. Current passing through thecoating flows through the steel to a contact made directly to the steelby means of a twist drill. The resulting amperage provides a measure ofthe resistance encountered through the coating. Several hundred contactsare employed in obtaining readings for coating evaluation. A completeshort circuit is indicated by a reading of milliamps per 0.1 squareinch.

For the coatings of the present invention, Franklin test values of about45 or less are considered acceptable. The results obtained for CoatingCompositions A and B respectively are as follows:

Franklin test milliamps/ 0.1 sq. in. Coating Composition A 26 CoatingComposition B 52 EXAMPLE 2 The procedure of Example 1 was repeatedexcept that the coating composition contained the following componentsin the indicated ratios:

Coating Composition C: Molar ratio MgOzSiO 1480:1 B O :SiO 4.6:1 MgOzNa2830z1 The procedure of Example 1 was repeated except that the coatingcomposition contained the following components in the indicated ratios:

Coating Composition D: Molar ratio MgO:SiO 1480:1 B O :SiO 8.7:1 MgOzNa2830:1

The coated annealed steel plates were subjected to the Franklin Testwith the following results:

Franklin test milliamps/ 0.1 sq. in. Coating Composition C 26 CoatingComposition D 58 EXAMPLE 3 The procedure of Example 1 was repeatedexcept that the ratio of B 0 to SiO employed was 4.621 to provide acoating composition having the following components in the indicatedratios:

Coating Composition E: Molar ratio MgOzSiO 1480:1 1320 18102 MgOzNa2800:1

composition having the following components in the indicated ratios:

Coating Composition F: Molar ratio MgOzSiO 1480:1 B203 I 2-3: 1 MgOzNa2800:1

The coated steel plates were Observed for uniformity of coating byvisual inspection. The results were as follows:

Molar ratio Uniformity of (Bros-'SiTz) coating 4.611 Excellent. 2.3:1Barespots.

Coating Composition E Coating Composition F EXAMPLE 4 The procedure ofExample 1 was repeated except that the ratio of MgO to sodium was variedwhile keeping the ratios of MgOzSiO and B O :SiO constant to providecoating compositions having the following components in the indicatedratios:

Steel plates were coated with Coating Compositions A, G and Hrespectively, and were examined visually for uniformity of coating withthe following results:

Uniformity of coating Coating Composition A Good.

Coating Composition G Poor.

Coating Composition H Poor.

EXAMPLE 5 A coating composition was prepared as in Example 1, exceptthat the molar ratio of MgOzSiO was as indicated:

Coating Composition 1: Molar ratio MgO:SiO 233021 B O 2SiO 3.5:]. MgO:Na2800:1

The procedure of Example 1 was followed in coating this composition ontosteel plates. A control consisting of MgO with no additives was alsocoated onto steel plates according to the same procedure. The coatedsteel plates were then tested for corrosion resistance by means of theChlorine Test for Corrosion Resistance.

Chlorine test for corrosion resistance In this test strips of weighedSteel having 30 cm? of surface area rest against a thermocouple welllocated at the center of the hot Zone of a heated Vycor tube in anautomatically controlled tube furnace. Helium flows through the tube ata linear velocity of 65 cm./min, The temperature is raised to 816 C. tosimulate stress relief annealing conditions and minutes are allowed forequilibrium to be established after the temperature levels off. The gasis then suddenly switched to wet chlorine moving at cm./min. After 60seconds, the flow is switched back to a 170 cm./min. fiow of helium. Thesystem is then cooled to 315 C., samples removed, quenched in coldrunning water, scrubbed, dried and reweighed. The Weight loss due toiron and silicon chloride vaporization is an inverse measure of thecorrosion resistance.

The results obtained were as follows:

A coating composition was prepared as in Example 1 except that sodiumtetraborate was used instead of sodium hydroxide and the molar ratio ofMgOzSiO was 943:1 to yield a composition having the following componentsin the indicated ratios:

Coating Composition J 2 Molar ratio MgO:SiO 943:0 B O :SiO 3.5:1 MgO:Na280021 The procedure of Example 1 Was followed in coating thiscomposition onto steel plates except that the coated plates wereannealed for a period of 8.6 hours. The coated, annealed plates weretested for corrosion resistance according to the procedure described inExample 5.

The results obtained were as follows:

Reduction in Weight weight loss, Percent loss, compared to improvegramscontrol ment Coating Composition J 0.21 0. 11 34. 4 C ontrol 0. 32

EXAMPLE 7 A coating composition was prepared as in Example 1(hereinafter coating composition K) except that sodium tetraborate wasused instead of sodium hydroxide and the molar ratio of MgO:SiO was2830:1 to yield a composition having the same ratios of components asthe composition of Example 5.

The procedure of Example 1 was followed in coating this composition ontosteel plates except that the coated plates were annealed for a period of8.6 hours, The coated, annealed plates were tested for corrosionresistance according to the procedure described in Example 5.

The results obtained were as follows:

Reduction in Weight weight loss, Percent loss, compared to improvegramscontrol ment Coating Composition K 0. 17 0. 15 46. 9 Control 0. 32

EXAMPLE 8 A composition (hereinafter coating composition L) was preparedby fusing together magnesium metaborate, Mg(BO and SiO to prepare afrit. The frit was then comminuted and suspended in an aqueous solutioncontaining sodium hydroxide and boric acid in such proportions to yielda composition having the same ratios of components as coatingcompositions I and K.

The procedure of Example 1 was followed in coating this composition ontosteel plates except that the coated plates were annealed for a period of8.6 hours. The coated, annealed plates were tested for corrosionresistance according to the procedure described in Example 5.

The results obtained were as follows:

A coating composition (hereinafter coating composition M) was preparedaccording to the procedure of Example 1 except that the molar ratio ofMgO:SiO- was 283:1 to yield a composition having the followingcomponents in the indicated ratios:

Coating Composition M: Molar ratio MgO:SiO 283:1 1320318102 3.511 MgOzNa280021 The procedure of Example 1 was followed in coating thiscomposition onto steel plates except that the coated plates wereannealed for a period of 8.6 hours. The coated, annealed plates weretested for corrosion resistance according to the procedure described inExample 5.

The results obtained were as follows:

Reduction in weight loss Percent compared to improvecontrol merit Weightloss, grams Coating Composition M Control EXAMPLE 10 A steel coatingcomposition (hereinafter coating composition N) was prepared by admixingmagnesium oxide, boric oxide, sodium tetraborate and silica in thefollowing molar ratios:

Coating Composition N: Molar ratio MgOISiO 148011 B O :SiO 3.511 MgOzNa280011 Reduction in weight loss compared to grams control ment Weightloss,

Percent improve- Coating Composition N Control While several particularembodiments of this invention are shown above, it will be understood, ofcourse, that the invention is not to be limited thereto, since manymodifications may be made, and it is contemplated, therefore, by theappended claims, to cover any such modifications as fall within the truespirit and scope of this invention.

What is claimed is:

1. A magnesium oxide composition containing a major proportion ofmagnesium oxide having a surface area of at least 20 square meters pergram and a minor proportion of the additives:

(I) a boron compound selected from the group consisting of boric acids,boric oxides, and magnesium borates;

(II) a sodium compound selected from the group consisting of sodiumhydroxide, sodium oxide, sodium peroxide and sodium 'borate; and

(III) a silicon-containing compound selected from the group consistingof silica and alkaline earth metal silicates;

wherein the molar ratio of magnesium oxide to silica is from about 280:1to about 2900:1, the molar ratio of boron expressed as (B to silica isfrom about 3:1 to about :1, and the molar ratio of magnesium oxide tosodium is from about 2000:1 to about 4000:1.

2. A composition according to claim 1 wherein the boron compound isboric acid, the sodium compound is 10 sodium hydroxide, and thesilicon-containing compound is silica.

3. A composition according to claim 1 wherein the molar ratio ofmagnesium oxide to silica is from about 100011 to about 2000:1, themolar ratio of boron expressed as B O to silica is from about 3:1 toabout 5:1, and the ratio of magnesium oxide to sodium is about 2000:1 to400021.

4. A composition according to claim 1 wherein the molar ratio ofmagnesium oxide to silica is from about 1000:1 to about 2000:1, themolar ratio of boron expressed as B O' to silica is from about 3:1 toabout 4:1, and the ratio of magnesium oxide to sodium is about 2000:1 toabout 4000:1.

5. A composition according to claim 4 wherein the molar ratio ofmagnesium to sodium is about 2000Il.

6. A composition according to claim 1 wherein the aggregate molepercentage of the additives boron, silica and sodium to the totalcomposition is up to about 2 /2 mole percent.

7. A composition according to claim 1 wherein the aggregate molepercentage of the additives boron, silica and sodium to the totalcomposition is up to about 2 mole percent.

8. A composition according to claim 1 wherein the molar ratio ofmagnesium oxide to silica is about 1500: 1, the molar ratio of boricacid expressed as B 0 to silica is about 35:1, and the molar ratio ofmagnesium oxide to sodium is about 2800:1.

9. A composition according to claim 1 wherein the boron compound isboric acid, the sodium compound is sodium tetraborate, and thesilicon-containing compound is silica.

10. A process for coating steel sheet with an electrically insulating,corrosion-resistant coating which comprises in sequence forming an.aqueous slurry of a magnesium oxide composition as defined in claim 1,coating said slurry to the surfaces of steel sheet, heating the slurrycoated steel to dry said coating, and thereafter annealing said coatedsteel sheet.

11. A process according to claim 10 wherein the slurry has a solidscontent of from 5 to 20 percent by weight.

12. A process according to claim 10 wherein the silica has a particlesize of up to 200 mesh (US. Standards Sieve Series).

13. A process according to claim 10 wherein the steel sheet is coiledafter coating and before the annealing step.

14. A process according to claim 10 wherein the boron compound is boricacid, the silicon-containing compound is silica, and the sodium compoundis sodium hydroxide.

15. A coated steel sheet having thereon an adhered coating of thecomposition defined in claim 1.

References Cited UNITED STATES PATENTS 1,703,630 2/1929' Meehan 14827X2,354,123 7/1944 Horstman et al. 14831.55X 2,515,788 7/1950 Morrill14814X 2,641,566 6/1953 Robinson 14827X 3,073,722 1/1963 Hoehn et al.14812.1X 3,084,081 4/ 1963 Carpenter 14814 3,105,781 10/1963 Walter148113X 3,186,867 6/1965 Forslund et al. 148113X 3,331,713 7/1967 Miller148--113 DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, AssistantExaminer US. Cl. X.R.

